1. Field of the Invention
This invention relates generally to balanced to unbalanced transformers (baluns) and more particularly to an improved Marchand balun adapted for use on printed circuit boards.
2. Background Art
In radio frequency integrated circuits, it is often desirable for the input and output connections to be differential. An example of a differential connection is two wires having an equal impedance to a common ground conductor, their respective signals 180 degrees out of phase. A transmission line having these characteristics is known as a balanced line, as opposed to an unbalanced line. The advantages of a balanced radio frequency signal input over an unbalanced input include higher dynamic range, higher bandwidth, and lower pick-up and generation of interference.
Unfortunately, many radio frequency components, such as coaxial cable, are unbalanced. An adapter is required to convert the unbalanced signal into a balanced one without loss or distortion and while maintaining the proper matching impedance to terminate the transmission line. A passive device that achieves this function is known as a balun, and can be constructed in various ways. Many existing passive baluns either are too large, too expensive, too complex or have an insufficient bandwidth to be effectively implemented in a printed circuit board RF application.
What is needed is a passive balun that exhibits low signal distortion, high bandwidth, low loss, and has a good impedance match. Additionally, the passive balun should be small, simple to manufacture, and tolerant of variations.